The invention is in the field of particle sampling, collection and analysis. The invention relates generally to devices and methods for sampling and characterizing particles in fluids including air and process chemicals (e.g., gases and liquids) for applications including the evaluation of contaminants in a range of cleanroom and manufacturing environments, and, more particularly, to airflow systems that provide precise volumetric airflow measurements to particle sampling systems.
Cleanrooms and clean zones are commonly used in semiconductor and pharmaceutical manufacturing facilities. For the semiconductor industry, an increase in airborne particulate concentration can result in a decrease in fabrication efficiency, as particles that settle on semiconductor wafers will impact or interfere with the small length scale manufacturing processes. For the pharmaceutical industry, where this type of real-time efficiency feedback is lacking, contamination by airborne particulates and biological contaminants puts pharmaceutical products at risk for failing to meet cleanliness level standards established by the Food and Drug Administration (FDA).
Standards for the classification of cleanroom particle levels and standards for testing and monitoring to ensure compliance are provided by ISO 14664-1 and 14664-2. Aerosol optical particle counters are commonly used to determine the airborne particle contamination levels in cleanrooms and clean zones and liquid particle counters are used to optically measure particle contamination levels in process fluids. Where microbiological particles are a particular concern, such as in the pharmaceutical industry, not only is quantification of the number of airborne particles important, but evaluating the viability and identity of microbiological particles is also important. ISO 14698-1 and 14698-2 provide standards for evaluation of cleanroom and clean zone environments for biocontaminants.
Collection and analysis of airborne biological particles is commonly achieved using a variety of techniques including settling plates, contact plates, surface swabbing, fingertip sampling and impactor-based active air samplers. Cascade impactors have traditionally been used for collection and sizing of particles. In these devices, a series of accelerations and inertial impacts successively strip smaller and smaller particles from a fluid flow. Each single stage of an inertial impactor operates on the principle that particles suspended in air can be collected by forcing a dramatic change in the direction of the particle containing airflow, where the inertia of the particle will separate the particle from the airflow streamlines and allow it to impact on the surface. Biswas et al. describe the efficiency at which particles can be collected in a high velocity inertial impactor (Environ. Sci. Technol., 1984, 18(8), 611-616).
In many cleanroom environments, retrieving size information from a particle impactor is not necessary. In this case, a single stage active air sampling impactor system is sufficient to collect biological particle concentrations subject to subsequent detection and analysis. In an impactor-based active air sampler used for collection of biological particles, the impact/collection surface commonly comprises a growth medium, such as an agar plate, as would be used with other biological particle collection techniques. After the particles are collected onto the growth media surface, the media is incubated to allow the biological particles to reproduce. Once the colonies reach a large enough size, they can be identified and characterized, for example using microscopic imaging, fluorescence, staining or other techniques, or simply counted visually by eye or by image analysis techniques.
For these types of biological particle collection and analysis techniques, various operational aspects are important to ensure efficient collection, detection and analysis. For example, the collection efficiency may be of high importance, as failing to detect that biological particles are present in cleanroom air can result in the cleanroom environment having higher levels of contamination than detected. Upon determination that under counting has occurred, pharmaceutical products made in those environments can be identified as failing to meet required standards, potentially leading to costly product recalls. Similarly, failing to ensure that the viability of collected biological particles is maintained during the collection process will also result in under counting. Such a situation can arise, for example, if the collected biological particles are destroyed, damaged or otherwise rendered non-viable upon impact with the growth medium, such that the collected particles do not replicate during the incubation process and, therefore, cannot be subsequently identified.
On the opposite extreme, biological particle concentrations can be overestimated due to false positives. Over counting of this nature arises where a biological particle that is not collected from the cleanroom air, but is otherwise placed in contact with the growth medium, is allowed to replicate during the incubation process and is improperly identified as originating from the cleanroom air. Situations that contribute to false positives include failing to properly sterilize the growth medium and collection system prior to particle collection and improper handling of the growth medium by cleanroom personnel as it is installed into a particle collection system and/or removed from the particle collection system and placed into the incubator. Again, this can result in a pharmaceutical product being identified as failing to meet required standards. Without sufficient measures to identify false positives, such a situation can result in pharmaceutical products that actually meet the required standards, but are destroyed due to an overestimation of biological particle concentration in the cleanroom air indicating that the standards were not met.
There remains a need in the art for particle collection systems capable of achieving efficient sampling of biological particles. For example, particle collection systems are needed for cleanroom and manufacturing applications that provide high particle collection efficiencies while maintaining the viabilities of collected bioparticles. In addition, particle collection systems are needed for cleanroom and manufacturing applications that reduce the occurrence of false positive detection events.
It is desirable in the field of particle collection systems to determine the number of particles for an exact volumetric flow rate (i.e., one cubic foot per minute, or “CFM”) as sampled from the ambient environment within the cleanroom or clean zone. By way of example, particle collection systems in cleanrooms typically assume that contamination levels are measured relative to a volume of air, and not relative to mass, since the mass of a given volume of air can vary by over 20% due to elevational changes alone. Weather fronts induce additional mass-per-volume differentials of up to 3%.
Therefore, one problem in conventional particle collection systems is that actual volumetric flow rate is calibrated for a given pressure, typically at “sea level,” such that particle counting at other locales does not reflect a correct volumetric flow rate. Manual adjustments can be made to compensate for these errors, except that continuous manual adjustment is needed to ensure accuracy over changing environmental (e.g., elevation and barometric pressure) conditions.